Why did the Japan declare war?

Answer 1

The Soviets always acted in their own self-interest in their foreign policy, which is in a way admirable for any government.

Let's keep in mind also that there was an existing track record: the Russo-Japanese war, several decades earlier.

With the war against the Japanese obviously within days of ending in WWII, the Soviet Kremlin government "declared war" not to be good boys on the block or trade for favors or make a good impression, but purely in a grab for the long-disputed smaller Japanese islands that the Soviet Union/Russia had either long wanted or long claimed already!

This was chiefly an opportunistic land grab, a territorial move, in the direct interest of the Soviet Union itself. Nothing more.

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The above is the textbook answer the the entire World is sold on...

In reality Russia was hours away from a full attack on Japan.

The latter is the PRIMARY reason USA dropped the bomb!

The faster USA was able to get Japan to surrender the less problems associated with dividing Japan, we learned our lesson when we allowed Russia to infiltrate Germany!

Answer 2

Why? Because they intended to fight.

If you really meant "When"; then usually Japan declared war on the day of the attack, both Russia (Russian-Japanese War 1904) and America (WWII) received their declarations of war on the day of the attack. The Japanese military used to practice one of the principles of war: Surprise.

Answer 3

USSR was part of the Allies, but mostly because Japan was spreading and their empire was becoming a threat to the USSR